Recombinant DNA clone containing a genomic fragment of PfHRP-II gene from Plasmodium falciparum

ABSTRACT

The present invention resides, in part, in cloned DNA fragments encoding a histidine-rich protein of Plasmodium falciparum (PfHRP-II). PfHRP-II is a malarial antigen that is secreted by the parasite into erythrocytes. The protein is characterized by tandem repeats of high histidine, alanine and aspartate content. The protein is found to have high affinity for divalent cations. Cloning and characterization of the PfHRP-II gene are described, as are antibodies against PfHRP-II.

This application is a divisional of copending application Ser. No. 07/518,299, filed on May 3, 1990, now U.S. Pat. No. 5,130,416 which is a continuation of Ser. No. 07/279,245 filed on Dec. 1, 1988 (now abandoned), which is a divisional of Ser. No. 06/895,942 filed on Aug. 13, 1986 (now abandoned). The entire contents of all of these applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention is related to malarial antigens and the genes encoding the same. More particularly, the present invention is related to a recombinant DNA clone (pDL4.l) containing a genomic fragment of PfHRP-II gene from Plasmodium falciparum.

2. State of the Art

Of various parasitic diseases, malaria remains one of the most menacing conditions. Substantial progress has been made in identifying the pathogenic factors causing malaria and Plasmodium falciparum has been recognized as the major pathogen of human malarial disease.

Recent advances in biochemical, immunological and genetic technologies have led to the recognition of certain antigens and to the characterization of certain genes or gene-fragments encoding the synthesis of malarial pathogenic antigens. The present invention relates to one such discovery.

SUMMARY OF INVENTION

An object of the present invention is to provide a new, soluble, histidine-rich protein, designated PfHRPII, from P. falciparum infected erythrocytes.

A further object of the present invention is to provide a recombinant DNA clone containing a genomic fragment capable of encoding PfHRP-II protein.

A still further object of the present invention is to provide a method of early detection or diagnosis of malarial infection employing the PfHRP-II antigen or an antibody having specificity against said PfHRP-II antigen.

It is another object of the present invention to provide a pharmaceutical composition comprising immunogenic amount of PfHRP-II to induce protective immunity in a host to which said pharmaceutical composition is administered in a pharmaceutically acceptable vehicle or carrier.

Other objects and advantages will become evident as the detailed description of the invention proceeds

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, features and many of the attendant advantages of the invention will be better understood upon a reading of the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 shows the genomic map and sequencing strategy for PfHRP-II. Coding regions are directed from left to right and are marked by heavy lines on the genomic maps. Comparison of the Alu I and Dra I restriction fragments from genomic DNA with those from λMABl revealed a 279bp deletion occurring within the region of tandem repeats but the reading frame having been preserved. Restriction patterns of inserts from clones pDL4.1 and pDL11.3 were compared with corresponding restriction digests of genomic DNA by Southern blotting and showed no evidence of deletion or rearrangement. Nucleotide sequence analysis was performed on subclones in M13mp18 having targeted deletions generated by timed exonuclease III digestions. The coding strand of the pDS11.1 insert was determined from fragments digested with Pvu II and subcloned into M13mp19. Sp, splice site at 3' end of intron. A, Alu I; D, Dra I; F, Fok I; H, Hinf I; N, Nde I; P, Pvu II; S, Ssp I;

FIG. 2 (A) shows the genomic and deduced amino acid sequences of PfHRP-II displayed by automated plotting. The register of the open reading frame of PfHRP-II is in phase with the gene for β-galactosidase in λMABl as expected, since this clone produces a fusion protein reacting with both McAb87 and anti-β-galactosidase. The sequence shown for PfHRP-II does not include the nucleotides added by ligation of Eco RI linkers (GGAATTCC) to the genomic fragment during library construction.

FIG. 2(B) shows the sequence of PfHRP-II mRNA obtained by primer extension analysis. The splice junction is indicated by the arrow. The hydrophobic leader sequence is underlined;

FIG. 3 shows the schematic representation of the gene for PfHRP-II. The lengths of the coding regions, represented by boxes, are drawn to scale. Intervening sequences (I) and repeat domains (R_(II)) are shown; and

FIG. 4 shows immunoprecipitation of PfHRP-II from infected culture supernatant with McAb87 (identified in FIG. 4 as M87) and rabbit antiserum (identified in FIG. 4 as RdP) to the synthetic oligopeptide AHH(AHHAAD)₂. The PfHRP-II is labelled with ³ H-alanine (A) and ³ H-histidine (H) but not with ³ H-isoleucine (I).

DETAILED DESCRIPTION OF INVENTION

The above and various other objects and advantages of the present invention are achieved by a recombinant DNA clone (pDL4.1) containing a genomic fragment of the PfHRP-II gene from P. falciparum.

Unless defined otherwise, all technical or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned hereunder are incorporated herein by reference.

The term "PfHRP-II" as used herein means a protein whose amino acid sequence and encoding genetic map (nucleotide sequence) are shown in FIG. 2A.

A. Methods of Isolation and Characterization of the Clone

1. A genomic expression library in the vector λgtll was constructed from Plasmodium falciparum DNA digested with mung bean nuclease. The description of this library and the methods of construction was the same as described by McCutchan et al. (Science 225:625-628). DNA from the 7G8 clone of the Brazil isolate IMT22 of P. falciparum (Burkot et al., Trans. R. Soc. Trop. Med. Hyg., 78:339-341) was used to construct the library.

2. A monoclonal antibody, McAb87, was prepared which reacts specifically with the histidine-rich protein PfHRP-II. The methods of preparation and characterization of McAb87 was the same as described by Howard et al. (J. Cell. Biol., 1986).

3. A recombinant DNA clone (λMABl) was isolated from the genomic expression library by immunoscreening with McAb87. The methods used in the immunoscreening was the same as described by Young and Davis (Proc. Natl. Acad. Sci. USA, 80:1194-1198; Science, 222:778-782) and Dame et al. (Science 225:593-599). Immunoblot analysis of the recombinant clone showed that λMABI produced an inducible fusion protein of approximate Mr 144,000 which reacted with both McAb87 and anti-β-galactosidase.

4. The insert was excised with Eco RI from λ-MABI DNA purified by gel electrophoresis and cloned into the sequencing vector M13mp18 using standard procedures (Maniatis et al., Molecular cloning: A Laboratory Manual, Cold Spring Harbor Laboratory, 1982; Yanish-Perron et al., Gene 33:103-119). Sequence analysis was performed on subclones in M13mp18 having targeted deletions generated by exonuclease III digestion. Sequences were obtained by the dideoxynucleotide method (Sanger et al., Proc. Natl. Acad. Sci. USA, 74:5463-5467) using commercially supplied reagents (Bethesda Research Laboratories, Bethesda, Md.).

The sequence revealed a region of tandem repeats extending from nucleotides 148-66g predominantly encoding the oligopeptides AHH and AHHAAD, wherein A =Alanine; H=Histidine and D =Aspartic acid. Restriction analysis identified a 544 bp fragment which spanned the repeats. This fragment was purified and used to probe DNA transfer blots of genomic DNA (Southern, J. Mol. Biol. 98:503). Comparison of the restriction fragments from genomic DNA with those from λMABl revealed that a 279bp deletion had occurred in the repeat region but had preserved the reading frame.

5. To obtain the complete region of tandem repeats in the PfHRP-II gene, P. falciparum DNA was restricted with the enzyme Dra I and ligated into the Sma I site of the plasmid pUC18 using standard methods (Maniatis, supra). Two clones, pDL4.1 and pDLll.3, were obtained having identical inserts oriented in opposite direction in the vector. Restriction analysis and comparative DNA transfer blotting were performed as described herein supra and showed no evidence of deletion or rearrangement.

6. RNA transfer blots were performed to confirm that the cloned DNA was transcribed. RNA was isolated from saponin purified P. falciparum parasites suspended in 100 mM NaCI, 10 mM tris pH 8.0, 2 mM MgCl₂, 10 mM vanadyl-ribonucleoside comlexes and 1% NaDodSO₄. Sequential extraction was carried out with hot phenol and chloroform according to the method of Hyde et al. (Mol. Biochem. Parasitol., 4:283-290). RNA transfer and hybridization with nick-translated pDL4.1 were performed according to the protocols described by Mehdy et al. (Cell 32:763-771).

7. The complete nucleotide sequence of each insert from pDL4.1 and pDL11.3 was determined by the dideoxynucleotide method of Sanger et al, supra, and both were found to be identical. FIGS. 1, 2 and 3 show the map and sequence obtained for the region of the PfHRP-II gene extending from the 3' splice junction of the intron to and through the stop codon.

8 The nucleotide sequence of the RNA transcript extending from the initiating codon to the intron splice junction was obtained by primer extension analysis according to the protocol of Belfort et al. (Cell 41:375-382). Oligonucleotide primers (synthesized as recommended by the manufacturer, Applied Biosystems DNA synthesizer, Foster City, California) were used which were complementary to nucleotides 64-83 and 114-135 of the PfHRP-II genomic sequence (FIG. 2). Reactions were carried out using 0.5 pmol of 5' labelled synthetic oligonucleotide and 10 μg of P. falciparum RNA.

9. To confirm the reading frame and deduced amino acid sequence of PfHRP-II as correct, antisera were generated against a synthetic oligopeptide containing repeats from the deduced sequence and used to immunoprecipitate biosynthetically labelled PfHRP-II. The synthetic oligopeptide AHH(AHHAAD)₂ was synthesized by the solid-phase method of Merrifield and Marglin (Ann. Rev. Biochem. 39:841-866) and cleaved from the solid support with liquid HF (Tam et al., J. Am. Chem. Soc. 105:6442-6455). The oligopeptide was desalted using a Biogel P-2 column and coupled to Keyhole Limpet hemocyanin (KLH). Rabbits were immunized with biweekly injections of a 1:1 mixture of the synthetic oligopeptide, AHH(AHHAAD)₂, and KLH in Freund's complete adjuvant and antisera were obtained during the 7th week following the first injection. The antiserum thus obtained and McAb87 reacted with specificity with the protein (PfHRP-II) obtained from infected erythrocyte, thus demonstrating the identical nature of the synthetic and the naturally occurring PfHRP-II epitope (FIG. 4). The observed specificity of reaction between the antigen (PfHRP-II) and the antibodies indicates neutralizing efficacy of the antibodies against PfHRP-II.

10. Studies of biosynthetically labelled cultures were performed on the 7G8 clone of P. falciparum maintained in vitro by the methods of Trager and Jensen (Science, 193:673-675). Labelled amino acids L-[2,5-³ H]-histidine, L-[2,3-³ H]-alanine, L-[4,5-³ H]-isoleucine and the labelled sugar D-[6-³ H]-galactose were obtained from Amersham Corporation (Arlington Heights, Ill.) and used for biosynthetic labelling according to the procedures of Leech et al. (J. Cell. Biol. 98:1256-1264). Cultures were harvested after 24 hours of labelling and immunoprecipitated by standard methods (Kessler, J. Immunol. 115:1617-1624) using McAb87 and the antisera described in step 9, supra.

A deposit of the recombinant DNA clone (pDL4.1) prepared in accordance with the present invention has been made at the American Type Culture Collection, Rockville, Md. under accession number 40248. It is noted that the deposit made at the ATCC shall be viably maintained for the life of the patent if issued or for at least 30 years from the date of the deposit and made available without restriction to the public upon issuance of the patent, of course, consistent with the provisions of the law.

B. Properties of the PfHRP-II Gene and Expressed Protein

1. The PfHRP-II gene has an interrupted structure, with an intron separating a short (69 bp) exon encoding a hydrophobic leader from a 927 bp exon encoding numerous tandem repeats of very high histidine, alanine and aspartate content (FIGS. 2 and 3)

2. The gene is transcribed to produce an RNA transcript of approximately 2.1 kb.

3. The nucleotide sequence encodes a protein of approximate molecular weight 35,138, a value much lower than the Mr of 60,000-80,000 obtained by SDS-PAGE. Without being bound to any theory, possible explanations for this difference include post-translational events (e.g. dimerization) and anomalous migration during SDS-PAGE .

4. The deduced sequence contains about 34% histidine, 37% alanine and 10% aspartate.

5. PfHRP-II migrates as a multiplet of bands generally spanning 5,000-10,000 Mr which is indicative of post-translational processing.

6. The protein is exported from the parasite into the body fluid. PfHRP-II passes through the host erythrocyte in concentrated "packets" and is released from the infected erythrocyte into the body fluid in vivo or into the culture supernatant in vitro. The mature protein recovered from culture supernatant corresponds to the slowest moving band of the multiplet reactive with McAb87.

7. The mature protein is glycosylated and incorporates radiolabelled galactose.

8. The protein exhibits strong binding to the divalent cations Zn++and Cu++(Table 1). Binding of Zn++ to PfHRP-II can be reversed by imidazole, the side group of histidine. Other cations forming chelation complexes with histidine (such as Cd, Hg, Co, Ni) would likewise bind with PfHRP-II.

9. PfHRP-II binds strongly to heparin-Sepharose (Pharmacia) and cannot be eluted by a gradient of NaCI up to 2M NaCl, whereas other proteins which bind to heparin-Sepharose are eluted by≦1.5 M NaCI, indicating the polycationic nature of multiple imidazole groups of PfHRP-II.

10. Using McAb87 and/or rabbit antisera, experiments with serum from patients infected with malaria demonstrated the presence of circulating PfHRP-II in infected blood and the presence of antibodies to PfHRP-II in previously infected patients (Data not shown).

11. PfHRP-II has a histidine content that is similar to that of a Mr 30,000 fragment obtained from a histidine-rich glycoprotein (HRG) that has been isolated from normal human serum (Morgan, Biochim. Biophys. Acta. 533:319). HRG interacts with divalent metal ions, heparin, thrombospondin, and autorosette-forming thymocytes. Levels of HRG are decreased in immunosuppressed states indicating that HRG may be linked to immune function. The properties common to PfHRP-II and HRG indicate similarity of the. functional role in vivo and that PfHRP-II may alter the physiologic role of HRG.

                  TABLE 1                                                          ______________________________________                                                             Position of elution                                                            with linear gradient                                                           of 0-750 mM imidazole                                                    Binding to                                                                             Peak width                                                                               Peak max                                       Protein         column    (mM imidazole)                                       ______________________________________                                         Elution of proteins from Zn.sup.2+ -chelated sepharose 6β by              imidazole                                                                      Bovine serum albumin                                                                           --        --        --                                         Human hemoglobin                                                                               +         10-40      25                                        Human transferrin                                                                              +          25-100    75                                        Human α2-macroglobulin                                                                   +          75-130   100                                        Human serum histidine-rich                                                                     +         100-180   140                                        glycoprotein                                                                   PfHRP-II        +         260-400   325                                        Elution of PfHRP2                                                              from Cu.sup.2+ -chelated sepharose 6B by imidazole                             PfHRP-II        +         ND        >450                                       ______________________________________                                    

In summary, the amino acid composition and the biochemical and biological properties of the PfHRP-II and its encoding gene clearly distinguish the recombinant clone from any other recombinant heretofore known.

Of course, the malarial antigen of the present invention prepared from the recombinant PfHRP-II clone or synthesized from the known amino acid sequence (FIG. 2) allows the preparation of a pharmaceutical composition comprising immunogenic amount of PfHRP-II to immunize against malaria in a host to whom said pharmaceutical composition is administered in a pharmaceutically acceptable vehicle or carrier such as physiological saline, nontoxic buffers, fillers or adjuvants and the like. Moreover, a kit comprising containers containing antigen and/or antibodies having specificity against PfHRP-II in suitable preservative medium such as physiological saline, nontoxic buffers and the like, and preferably lyophilized cryopreserved, can be utilized by standard immunological assays, well known in the art, to detect or diagnose even low level or early malarial infection which otherwise cannot be detected by conventional methods such as thin and thick blood smears and the like. The kit may also include such standard items as microtiter plates, micropipettes, agglutination reading means and the like which are normally found in such kits. It is understood that the examples and embodiments described herein are for ilIustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and the scope of the appended claims. 

We claim:
 1. A kit for detecting malarial infection comprising separate containers containing either PfHRP-II antigen or antibodies having specificity against PfHRP-II antigen in preservative medium.
 2. An antibody which specifically binds with an antigen of plasmodium having the following amino acid sequence ##STR1##
 3. The antibody of claim 2, which is a monoclonal antibody.
 4. The monoclonal antibody of claim 3, which is McAb87.
 5. The antibody of claim 2, which is a polyclonal antibody.
 6. The method for diagnosing malaria in humans which comprises:contacting the antibody of claim 2 with a sample from a patient suspected of having malaria; and determining if said antibody reacts with said sample.
 7. The method of claim 6, wherein said sample is serum.
 8. A method for detecting an antigen having the following amino acid sequence ##STR2## which comprises: contacting a sample suspected of containing said antigen with antibody of claim 7; anddetermining if said antibody reacts with said sample.
 9. A kit for the diagnosis of malaria comprising: an antigen having the sequence ##STR3## packaged in a container.
 10. A kit for the diagnosis of malaria comprising the antibody of claim 2, packaged in a container. 